use strict;

my $date;
my $lastdate;

while (($date = <DATA>) or $lastdate){
   $lastdate ||= $date;
   next if $lastdate eq $date;
   # print $lastdate report here
   print "Line $. ends date $lastdate";
   $lastdate = $date;
}

##</code><code>##

__DATA__
10
10
10
11
11
11
12
12
12

Line 4 ends date 10
Line 7 ends date 11
Line 9 ends date 12